Prius External add on Battery Pack.

I sort of like the add-on design of Enginer INC Plug-in Hybrid Electric Vehicle Prius Battery pack via using a 120 volt to 48 volt converter to charge the LiFePO4 pack of 50 volt +/- then taking the low voltage and running into main traction pack. From what I read on Inverter it converts it from 48 volt DC to 250 Volt DC. The Prius pack is 28 Modules of 7.2 volts equaling 201.6 volt DC nominal. The 168 cell pack @ 1.2 volts equals 201.6 volt to verify numbers. The add on kit is sold through Enginer, Inc in Troy MI. I am trying to find more info about the Inverter for the numbers don't quite add up. A 1.2 volt cell can be held to about 1.35 or 1.4 volts? and drawn down to 1.0 or 0.99 volts right? So wouldn't you charge it to 235.2 to maybe 226.8 volts exactly.
Also why such a low voltage design on using 48 volt to 250 volt inverter and using of 2 sets of 20 AH battery pack in series parallel design.
A very nice price for the kit from $3,495.00 to $4995.00
I am trying to get information my my neighbor who just a got a used Prius and want to increase MPG to 99 or more. I saw this at a green fair in Amherst MA and thought I'd see what else is out there for conversions other than adding a new 168 cell pack to increase range for PURE electric operation. I think 2005 is type three battery pack?